Antenna for communication with a satellite

ABSTRACT

A telecommunication system uses a satellite antenna showing on the horizontal plane, a sufficiently narrow radiation lobe to discriminate two adjacent satellites and showing on the vertical plane, a sufficiently narrow vertical lobe to ensure an adequate gain, but sufficiently ample to enable the reception of a certain number of satellites in adjacent orbital positions, by land stations distributed over a certain meridian arc without the necessity for antenna elevation angle variations.

This invention refers to telecommunication systems and it concerns inparticular a satellite telecommunication system.

Within the sphere of telecommunications, stationary satellites cover anever more important role, both for television connections and for allthe other applications, where they are used as transponders, for examplein telephone communications.

As it is known, these satellites are located at a height of approx.36,000 km in an orbit contained in the plane passing by the terrestrialequator. At this height, the speed, which this satellite must have tokeep its position in the orbit, nullifying the gravitational pull forcecoincides with the earth rotation speed. The final result for anobserver on the Earth, is that of seeing the fixed satellite in aprecise position on the horizon. The choice of the orbital position(15°East, 28°West etc.) is made to stay, as much as possible, over theregions on the earth surface where the television signal is to be sent.By way of example, the position of one of the Hot Bird™(13°East)satellites, used to broadcast television signals, is optimal to coverCentral Europe. Similarly, orbital positions shifted westwards are usedfor the Americas, and orbital positions shifted eastwards for Australia.

To ensure the best quality of signals received by earth stations,satellites are equipped with antennas having a radiation pattern shapedto concentrate transmitted power towards areas to be served.

Satellite position on the celestial vault, as seen by the land stationis defined by the azimuth and the elevation angles, referred to thegeographic North and to the horizon plane respectively, and it obviouslyvaries according to the geographical co-ordinates of the land stationitself. In particular, moving towards the poles, the angle between thehorizon and the satellite direction, is gradually reduced, while thelatitude increases (both Northwards and Southwards).

For this reason, to receive a stationary satellite in the regions nearthe Equator, it is necessary to aim the antenna almost to the zenith(should the orbital position not coincide with the zenith of the landstation, this angle must be recovered by slightly tilting the antenna).

Referring to a nation like Italy and to a stationary satellite like HotBird (13°East), the elevation required at Bolzano is of 36.3° and atRagusa it is of 47.1° (example of maximum and minimum latitude forItaly), with a total excursion of approx. 10 degrees. The use ofantennae with a sufficiently symmetrical radiation diagram with respectto the axis, entails therefore the necessity of having to set, at thetime of the installation, both the elevation and the azimuth. This iscurrently performed at the installation of antennae such as parabolas,both at a position that is fixed or a position that can beremote-controlled by means of mechanical positioning, see by way ofexample Patent EP0838876 application, or electronic controlled antennaeat array with scanning beam, see by way of example U.S. Pat. No.6,184,827.

Were it is possible to manage just the azimuth angle, installationoperation would be simpler, and hence cheaper, and it would be possibleto aim at different satellites without changing the elevation.

These problems are solved by the satellite communication system, beingthe purpose of this invention, and since it does not require aiming theantenna elevation, it can be installed in any place in a nation likeItaly and it therefore enables using, and hence producing anddistributing only one antenna model, entailing the reduction ofproduction and storage costs.

The antenna can be installed in a vertical position, adhering to a wall,and this therefore means that it is not so showy, and that it complieswith the regulations protecting the inner city urban decor.

A satellite telecommunications system as described in the characterisingpart of claim 1, is the particular purpose of this invention.

These and other characteristics of the present invention shall beclarified better by the following description of a preferred embodiment,given only by way of non limiting example, and by the enclosed drawingsin which:

FIG. 1 represents the main lobe of a radiation diagram of an antennarealised according to the invention;

FIG. 2 represents a possible embodiment of an antenna realised accordingto the invention;

FIG. 3 represents a power divider and an antenna supply network phaseshifter.

The system being the purpose of the invention includes a satellitereceiver, to which a set for displaying television programmes can beconnected, and an antenna, which as usual shows a sufficiently narrowradiation lobe on the horizontal plane, to discriminate two adjacentsatellites. As it is known, television satellites are as a matter offact spaced from one another by a minimum angle of approx. 3 degrees.

On the other hand on the vertical plane, the antenna shows a lobe whichis:

-   -   Wide enough to enable the reception of a certain number of        satellites in adjacent orbital positions, by land stations        positioned on a certain meridian arc, without the necessity for        changing the antenna elevation angle;    -   Sufficiently narrow to ensure an adequate gain, by way of        example of 30 db.

FIG. 1 schematically shows the main lobe of the required radiationdiagram, in side view a and in front view b, emitted by a plane surfaceantenna R. Plane p p cuts the lobe so as to highlight section p-p,having a vertical amplitude VA greater than the horizontal amplitude HA.

A planar antenna having such a radiation diagram, applied to aconvenient external wall which “sees” the satellite, can enable thereception of the satellite transmissions after orienting, mechanicallyor electronically, the main lobe in the direction of the concernedsatellite, acting only on one degree of freedom, that is the azimuth.

For what concerns both the movement and the shape of the radiationdiagram, it can be useful to resort to array antennae with scanning beamtechnology. These antennae have a planar structure and are achieved witha large number of radiant components, all of which are equal and equallyoriented. They are individually supplied with proper amplitude and phasesignals, in order to obtain a radiation diagram showing the main lobecomplying with the required elevation and azimuth direction.

The available project tools enable achieving an antenna with the mainlobe in the direction of a wide range of azimuth and elevation angles.The lobe itself can moreover be modelled to show the required amplitudeson both the horizontal plane and on the vertical plane. By way ofexample, a suitable antenna can be used in a nation like Italy, coveringa similar meridian arc and showing an equal average latitude, can show amain lobe amplitude on the vertical plane of approx. 10° and on thehorizontal plane of 1 to 2°.

Each individual antenna radiant element is placed according to lines andcolumns in matrix structure, and it is supplied by proper amplitude andphase coefficients through transmission lines arranged according tocolumns.

Calculation of such coefficients sets the main lobe elevation andamplitude angle, which must be similar to that in FIG. 1.

Each column of elements makes up a vertical sub-array featured by a setelevation angle, which can be repeated in an identical way all over thewhole antenna.

All the transmission line inputs supplying the aforesaid sub-arrays arecombined in just one input in order to obtain the required main lobedirection on the azimuth plane. There are different techniques forperforming this combination. Should just one direction be sufficient, afixed controller can be used, while should rather the scanning beperformed on the azimuth plane, numerical, electronic, RF devices etc.can be used.

The antenna can conveniently be achieved by using the micro-striptechnique, according to which both the radiant elements and the supplyleads can be made up of metal pads having a more or less wide or thinshape, achieved on a dielectric support.

FIG. 2 shows a possible embodiment of the antenna. The radiant structureis achieved on the rectangular plane surface R and it consists of atwo-dimension planar array of 8×8 radiant elements on micro-strip P,arranged to make a regular matrix structure. Horizontal spacing Sobetween the elements is not necessarily equal to the vertical spacingSv.

All the radiant elements belonging to the same column C, achieve asub-array, supplied by only one line in micro-strip and individual powerdividers and phase shifters for each element, in order to generate aradiation diagram, similar to that shown in FIG. 1 in the verticalplane.

FIG. 3 shows two basic devices of the antenna supply network. They are apower divider PD, provided with one input I with two outputs O1 and O2at different power sizes, and a phase shifter PH, introducing a phasedisplacement φ along path L.

The just described satellite antenna is only one of the possibleachievements suitable to the system being the purpose of the invention.The same functionality can be obtained by means of other technologies.By way of example, two further achievements are proposed. The firstachievement consists in actuating the radiant elements by means of hornssupplied by proper wave-guides. The second achievement, in lieu of thetraditional parabola uses a reflector antenna shaped to generate adiagram similar to that in FIG. 1 and driven by a simple poweredpositioner to select the azimuth and hence the required satellite.

Obviously the above descriptions are given by way of non limitingexamples. Variants and amendments are possible without for this exitingthe claim protection field.

1-12. (canceled)
 13. A satellite telecommunication system comprising: asatellite receiver and an antenna capable of ensuring communicationswith just one satellite in a stationary orbit, said antenna comprising aradiation lobe on the horizontal plane with a first amplitude todiscriminate two adjacent satellites, and a lobe on the vertical planewith a second amplitude to enable the reception by land stationsdistributed over a certain meridian arc of a certain number ofsatellites in adjacent orbital positions without the necessity ofchanging their elevation angle, and such as to ensure an adequate gain.14. The satellite telecommunication system according to claim 13,wherein said antenna is a planar structure antenna.
 15. The satellitetelecommunication system according to claim 14, wherein said antenna isan array antenna with a scanning beam, where the direction of the mainlobe can be directed at azimuth and elevation, and modelled to show saidfirst amplitude on the horizontal plane and said second amplitude on thevertical plane.
 16. The satellite telecommunication system according toclaim 15, wherein said antenna includes an array of radiant elements,all of them equal and equally oriented, individually supplied byconvenient amplitude and phase signals, in order to obtain a radiationdiagram showing a maximum irradiation in the required direction.
 17. Thesatellite telecommunication system according to claim 16, wherein eachof said antenna individual radiant elements is arranged according toline and column matrix structure and is supplied with convenientamplitude and phase coefficients through transmission lines arranged incompliance with the columns, the coefficients setting the elevationangle and amplitude of the main lobe.
 18. The satellitetelecommunication system according to claim 17, wherein each column ofantenna individual radiant elements making up a vertical sub-array,shows a certain elevation angle, and is identically repeated all overthe whole antenna.
 19. The satellite telecommunication system accordingto claim 18, wherein all transmission line inputs supplying saidsub-arrays are combined into one input in order to achieve the requiredmain lobe direction on the azimuth plane.
 20. The satellitetelecommunication system according to claim 16, wherein said radiantelements are horns supplied by convenient wave-guides.
 21. The satellitetelecommunication system according to claim 13, wherein said antenna isa shaped parabolic reflector antenna driven by a powered positioner. 22.The satellite telecommunication system according to claim 13, whereinsaid antenna shows a main lobe amplitude on the vertical plane ofapprox. 10° and an amplitude on the horizontal plane from 1 to 2°. 23.An antenna capable of ensuring communications with just one satellite ina stationary orbit for a satellite telecommunication system, saidantenna comprising: a radiation lobe on the horizontal plane with afirst amplitude to discriminate two adjacent satellites and a lobe onthe vertical plane with a second amplitude to enable the reception byland stations distributed over a certain meridian arc of a certainnumber of satellites in adjacent orbital positions without the necessityto change their elevation angle and such as to ensure an adequate gain.